To understand the process of pattern development in PWS or mokume Gane, you must first think in terms of manipulations to the billet. (I'm doing this on my phone with only one finger, so this could take a while.)

Manipulations are ways to rearrange or deform the starting billet of material. The first manipulation is the original stack configuration. The last is always the grinding of the billit to final shape. In be tween these two, are the steps of pattern development.

Original stack: Most folks start out with a simple stack of two different materials and alternate one after another. Whether the two materials are the same thickness, affects the final pattern. Whether different thicknesses of the same materials in the same stack, just in different locations, affects the pattern as well. Let's say we are making a PWS. Billet from 1095 (A) and 15N20. (B) The simplest of stacks uses equal numbers of layers with each layer equal thickness. So, both materials are an 1/8" thick and we arrange a 10 layer stack as ABABABABAB. if our B material is thinner than our A material, and we do the same stack, our pattern will be different than the one where all layers are equal thickness.

Try to visualize the first option. Equal numer of layers, each the same thickness. Now slice a diagonal line through it. You just cut through a layer cake and each layer appears the same as those adjacent to it, only a different color. Do the same thing where the B material is thinner than the A material and the layer cake looks slightly different.

Playing with the starting stack and the thicknesses of materials and the locations of those materials changes the pattern. If you still use only two materials, but you have some A that is 1/4" and some that is 1/8" and you have some B material that is 1/8" and some that is 1/16", how you arrange those layers in that stack affects the pattern when all is said and done.

The grind: The last manipulation is the grind. Whether that grind is flat, concave or convex (or changes along the length of the billet) changes the way the final pattern looks. How deep you grind also changes the pattern. Look at the post in Video and Mutimedia by Niels Provos for "Pattern Welding Explained". (It is pinned and for good reason) This shows how depth of grind through a simple single twist changes the pattern. https://www.bladesmithsforum.com/index.php?/topic/37018-pattern-welding-explained/

Intermediate manipulations: there are infinite manipulations and manipulation sequencing combinations to use in pattern development. Each one results in a different deformation of the billet. Each type of manipulation also can be done to varying degrees to make slight alterations to the pattern. To use an example, let's take a simple twist. How tightly you twist that billet affects the pattern when you grind into it. How deep you grind also changes the final pattern.

See how quickly this can expand your options with minor manipulations?

The simplest manipulation we do is forging the billet. You might think that's not much of a deformation or manipulation, but let's consider what happens during forging.

A hammer strikes a single spot in the billet and leaves a small dent or crater. It is deeper in the center than it is at the edges of the dent. So if I cut a cross section through the dent I would see a series of flat lines that gradually depressed and then gradually rose again to flat lines. Now repeat the hammering across the width and length of the billet. No two hammer blows are identical and most will overlap causing a multitude of dents in the surface, dents in the edges of other dents and dents in the edges of dents on dents, etc. The random denting causes the layer cake to compress in various locations to various depths. The resulting pattern when ground down can be quite stunning.

Types of manipulations: The most commonly used is probably the twist. There is drilling holes. Drill through a few layers in random locations and forge down so the bottom of the hole comes up to meet the top. The result is called either bird's eye or pool & eye. It's a bunch of little bullseye's in the surface. There's crushing, which is forging the edges down rather than the faces. This is also done by "forging on the bias" or truncating the corners of the billet to resquare it. Think tipping the billet so a corner is up and hammering it down. Repeat on all four corners until it is square again. Gary Mulkey does this in several of his posts so look for his WIP threads. They are excellent photographic records of pattern manipulation. The list of manipulation types is much longer and you are only limited by your imagination and willingness to add "one more manipulation".

Stacking or sequencing manipulations: This is using different manipulations in a sequence. So let's talk about twisting and forging. If you twist first and forge second, you will get a very different pattern than if you forge all those dents in and then twist. The pattern will look vastly different if you crush first and then twist and then tile cut. ( I didn't cover tile cutting did I? Well check out Gary's threads for how that's done)

So how does one determine what sequence to use to get a desired effect? Well, this takes two things: the ability to think backwards and the experience learned from experimentation.

Types of patterns. I break the different patterns into two families: Face grain patterns and End grain patterns. These refer to the position on the billet where the pattern is developed.

I can't think of any patterns that develop on the edges, except those that end up with edges the same as faces (like a twist) or ones where the edges are changed to ends (like feather) bit more on that later.

For now let's talk developing face grain patterns. We already discussed the random pattern and touched on the pool & eye. One more bit about P&E pattern. It can be achieved two ways: drilling holes and flattening the bar or pressing divots into the faces and grinding the bar down. I prefer starting very thick, drilling and forging down, drilling again and forging down and repeating until the desired thickness is reached.

Ladder patterns. This is a classic and ancient pattern. Like P&E, it can be achieved two ways, each of which has variations. So there are many different looks to a ladder pattern. Basically, it is developing a series of grooves across the faces, offset from one another and flattening the bar. You can press in the grooves and grind down, or you can cut the grooves and forge flat. The grooves can be wide or narrow, deep or shallow, close together or spaced apart, rounded or V shaped. All these things affect the topography of the final image in the pattern.

Twist Patterns. The twist is also an ancient pattern. The bar is drawn out to a long square bar and twisted. Some people grind the corners off before twisting some twist square, some forge the bar into a long round before twisting. Tight twist or loose twist, different looks happen. All these things change the topography in the pattern. Twists are often made with small cross section bars (3/8" to 3/4") and stacked side by side or in a bundle and forge welded back together to make a bar big enough to create the finished product. Alternating direction of twisted bars next to each other is commonly called a Turkish Twist pattern. Cutting a twist length wise in half and flipping it "inside out" yeilds an interesting look.

Cutting it in four pieces lengthwise and flipping all 4 pieces inside out is one version of explosion pattern. ( Sorry, no pics of that. Use your imagination)

End Grain patterns. These have become quite popular in the last few years with mosaics and W-pattern variations. I also include "canned patterns" in the end grain family. These are stacking various items like small billets, round rods, square rods, foil sheets, etc inside a square tube and filling the voids with powered steel. Weld it solid and cut off the can. You now have a solid bar with the pattern in the end grain.

What happens here is the end looks like a bunch of W's in rows of varying sizes facing each other open end to open end and point to point. You can get the end pattern on to the face by tile cutting (see Gary Mulkey threads) or by accordion cutting.

This is very similar to what you do in ladder pattern. Changing the depth and angles of the cuts drastically changes the topography of the pattern

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Joshua asked me to post this in this thread so here goes.
Just the spec.
This is a 340 layer random billet of high M 1075 and 15n20
Pattern is created by grinding into the final billet several times in the reduction process.
So once when the billet has been final welded and then once at about half reduction.
I grind at angle to each other making various crosses and X's.
This is coffee etched at 500 grit polish.